There could be a lot of reasons. Whiskers can cause this problem. Some stray dust could cause it. Leaky capacitors could cause it. Metal fragments in the case could cause it.
But ultimately, the cause was that something was short circuited. The short may burn through if it is just a small thing like a whisker or dust. In more severe cases, delicate circuitry susceptible to shorts is protected with a fuse. It could have been a fuse that burned through and not an IC, and the short could still exist (and remain a problem if the fuse failed). In extreme cases, the short may not result in a magical self-fix or a failed fuse and will instead remain shorted ultimately causing significant computer damage and possible household damage including fire.
Electrical fires are not to be trifled with. If you experience the "magic smoke" from a computer, I can only recommend that you forever unplug it and don't trust it in the future. If you absolutely have to power it back up, at least do so in a fireproof environment (an outdoor brick or concrete patio, drive way, or car park would be a good place) on a circuit that is protected by a household fuse or circuit breaker.
Peace,
Drew